High melt viscosity is a requirement for certain thermoplastic fabrication techniques such as extrusion blow molding, injection blow molding, profile extrusion, pipe extrusion, blown film extrusion, co-extrusion (with a second plastic material) extrusion coating, foam extrusion, foam molding, thermoforming, and the like, all require the thermoplastic composition to have a high melt viscosity and melt strength (melt elasticity) during processing.
Blow molding is used to make hollow shaped plastic articles in a variety of commonly encountered forms, such as milk bottles, auto windshield washer tanks, street light globes, arms and legs on toy dolls, and a multitude of others. There are two basic types of blow molding processes, both being fundamentally related, but technologically dissimilar. Extrusion blow molding typically comprises extruding a tube of plastic into a water-cooled mold, inflating the tube by internally introducing air or another gas until the walls of the molten tube assume the shape of the mold, allowing the shaped tube to cool to structural ridigity, and removing the extrusion blow molded part from the mold.
Another technique, injection blow molding involves instead of extrusion, injection molding the plastic around core pins in an injection mold, then transferring to the blow mold. The fundamental difference between injection blow molding and extrusion blow molding is that with the former, two complete sets of molds are required -- an injection mold for molding the preform and a blow mold for the final form.
Until now, polyolefin resins have been the plastics of choice for extrusion blow molding and while injection blow molding can use a greater variety of resins, even including poly(vinyl chloride) resins, not all of them can be substituted into the extrusion blow molding process because of a fundamental lack of melt strength and elasticity.
Most thermoplastics, and even polyolefins, have their shortcomings in extrusion blow molding, expecially if the preformed molten tubes (parisons) are too big and heavy. The tubes stretch and become difficult to handle and special equipment is needed to prevent this hot stretch. Moreover, the parts must often be removed from the blow mold while still warm and supported on special "cool-down" fixtures to avoid tearing, etc.
Other "blow" techniques have in common with the injection blow molding process that the molten polymer is inflated (by air, or a suitable inert gas) to assume its final desired shape. In extrusion blow molding an extruded tube is inflated inside a mold; in blown film extrusion an extruded tube is continuously inflated to a large diameter tube of low wall thickness, which is subsequently collapsed and further processed to yield film, (grocery) bags, etc., and in foam molding or foam extrusion applications a cellular structure is introduced in the plastic through expansion of an inert gas, again requiring high melt elasticity to prevent collapse of the foam before the part has solidified.
In extrusion of profiles of closely controlled shape and dimensions it is important that the molten plastic upon leaving the extruder die does not sag or drool until it has hardened or solidified.
In thermoforming, a sheet of plastic is suspended horizontally over a suitable mold and heated, usually by radiant heat, until melted. The sheet is then brought into contact with the mold and collapsed onto it by suction. After cooling, the plastic, which has assumed the shape of the mold is lifted off, trimmed and decorated as desired.
All of the above described methods require a thermoplastic that has a high melt viscosity which enables the parison, or extrudate to retain its shape until it is properly formed and solidifies via cooling. A solid state polymerization process provides for limited chemical interaction between blended polyesters, while at the same time causing polymerization to continue to cause an increase in molecular weight and melt viscosity of the product. This leads to the formation of block-copolymers having desirable physical and crystallization properties. Melt polymerization, instead of solid state polymerization, results in extensive interaction between polyesters in a blend, leading to random copolymers of greatly reduced crystallinity. Also, blends of polyesters generally have inferior physical properties due to the inhomogeneity of the material.
It has now been found that a high melt viscosity thermoplastic block co-polyester may be formed by melt blending two low to medium melt viscosity polyesters, and thereafter, subjecting the melt blended polyesters to a temperature that is below the melting component of the blend.
Accordingly, it is a primary object of this invention to provide a novel method of producing a high melt viscosity thermoplastic block co-polyester of two different polyesters that is useful for blow molding, extrusion blow molding and related applications.
It is also an object of this invention to provide a novel method of producing high melt viscosity thermoplastic block co-polyesters that include linear polyesters and branched co-polyesters that have a high melt strength which is advantageous for blow molding, extrusion blow molding and related applications.